It is known, in the back rest of a seat or in a bed, to provide a frame having a load-supporting means in the form of a membrane or a plurality of filaments extending under tension, between two opposite sides of the frame. Such a frame in a vehicle seat is normally upholstered and the lumbar support can be provided by shaping the upholstery in the lumbar region of the seat occupant. It is difficult however to vary the position of such a lumbar support to suit different seat occupants.